Process of activating cathodes



March 10, 1942. J BQND'LEY AL 2,275,886

PROCESS OF ACTIVATING CATHODES Filed April 50, 1941 OX/OE 60A TED CATHODE FAB/W624 T/O/V CA THODE EOUNDA T/ON 60A TE 0 W/ TH I ALKAL/NEEARTH HYD/POX/DE UNDER NON "10X lD/Z ING COND/T/ONS HYDROX/DE CONVERTED7'0 02005 [650 -a00 "0/ NON-OX/D/Z/N6 ENVIRONMENT PREFERA BLY HYDROGENCA THODE MOUNTED /N 0/5 CHARGE TUBE Inventors: Ralph J. Bondley, Herrnan A.Liebha1sky, Arthur F Winslow,

Then" Attorney.

Patented Mar. 10, 1942 PROCESS OF ACTIVATIN CATHODES Ralph J. Bondley,Scotia, Herman A. Liebhafsky, Schenectady, and Arthur F. Winslow,Scotia, N. Y., assignors to General Electric Company,

. a corporation of New York Application April 30, 1941, Serial No.391,090

7 Claims.

The present invention concerns the fabrication of electrical dischargedevices and is particularly directed to the preparation of coatedcathodes for such devices.

It is the object of our invention to improve the performance and reducethe exhaust schedule of discharge devices containing coated orWehnelt-type cathodes.

As is well known, the electron emissivity of cathodes is greatlyenhanced by the presence thereon of alkaline earth oxide such, forexample, as the oxide of barium. In accordance with a proceduredescribed in Schumacher United States Patent 1,467,398, cathodes forvacuum tubes are coated by dipping them into alkaline earth hydroxidewhich has been melted in the presence of water of crystallization.Partial oxidation of the cathode to be coated is'brought about by eitherpreliminary oxidation of the cathode before coating or by oxidation ofthe cathode in the coating bath which is operated at about 600 C., atwhich temperature it becomes an oxidizing medium.

We have discovered that marked improvements in the life and functioningof cathodes are obtained if the process of preparing the coating of suchcathodes is carried out under conditions whereby oxidation of thefoundation surface is avoided. We havealso discovered that markedadvantages are obtained by carrying out or completing the conversion ofthe hydroxide to the oxide as a separate step prior to the mounting ofthe coated cathodes in vacuum tube or other discharge device.

The accompanying drawing is a diagram of the main steps of the processconstituting our invention.

In accordance with our invention cathodes are coated initially with afusion of hydroxide of alkaline earth metal containing little or nowater of crystallization, preferably maintaining the temperature of suchfusion below 500 C. and causing or completing conversion of thehydroxide coating on the cathode to the oxide by heat treatment in anon-oxidizing environment, such as a protective atmosphere. A hydrogenatmosphere is preferred as it not only is protective in the sense ofbeing non-oxidizing but may have a positive reducing effect. Thehydrogen should be dry, that is, substantially free from water vapor.

Other features of novelty .will be pointed out in connection with thefollowing more detailed description.

In carrying out our invention, cathodes of clean, unoxidized tungsten,nickel, molybdenum, tantalum, or other suitable foundation material arecoated initially with alkaline earth hydroxide free from, or in anyevent containing no more than, a minimum amount of water ofcrystallization. Conveniently the form of hydroxide containing onemolecule of Water of crystallization in the molecule rather than theform containing 8H2O is employed. Hydroxide of barium, strontium,calcium or a desired mixture of hydroxides may be applied on a cathodeby dipping the cathode to be coated into a bath of hydroxide which ismaintained at a fusion temperature which is preferably below 500 C. Attemperatures of 475 to 500 C. we have found that little or no oxidationof the foundation metal occurs. At these temperatures some conversion ofhydroxide to oxide occurs, in the bath. As a result of dipping thecathode into such melt, a coating of the hydroxide (with or withoutoxide admixture) is caused to adhere to the foundation metal. Thecoating step may be carried out in the open air although the presence ofan enveloping atmosphere of protective gas, for example nitrogen orhydrogen, is beneficial.

Alternatively, in some cases when cathode configuration permits, theinitial coating of the foundation metal may be carried out by fusing thehydroxide in situ on the cathode. For example, a pellet of desiredhydroxide, or mixture of hydroxides, may be placed on a cathode having asurface capable of holding a pellet. Alternatively, hydroxide inpowdered form may be caused to fall on a heated cathode. When heatrequired for fusion is applied from any convenient source, that is,either from a separate heater or by causing the cathode itself to becomeheated by passage of current, the hydroxide melts and coats the surfaceof the cathode.

The cathode which has been coated with hydroxide under conditionsavoiding oxidation next is heated in accordance with the second step ofour process to a temperature at which conversion from the hydroxide tothe oxide occurs. This step preferably should be' carried outpreliminary to mounting the cathode in the discharge tube in which it isdesired for use and in a non-oxidizing environment, preferably in thepresence of a hydrogen gas at atmospheric pressure. For example, thehydroxide-coated cathode may be heated during the conversion step forabout one minute at a temperature of about 650 to 800 C. It is thenpreferably, but not necessarily, heated for a shorter time to a highertemperature, say to about 900 to 1100 C.

for During this heat treatment step v the hydroxide is dissociated intooxide and water vapor, the latter being carried away by convectioncurrents in the hydrogen. Other protective gases such as argon or heliummay be usedbut-dry hydrogen is preferred; and his desirable that aflowing stream of current of the electrical discharge device in which itis intended to function; The cathode coating-produced by the heattreatment step in the presence of a protective gas is surprisinglystable even in the presence of air under ordinary conditions. However,if a lengthy interval intervenes between its production and mounting ina vacuum tube it should be preserved in a vacuum or pro-. tectiveatmosphere, or be protected by'a suitable lacquer.

Although it is not necessary to convert the hydroxide completely to theoxide during the conversion step, the residue of unconverted hydroxideshouldnot be great enough to require significant physical change of thecoating during the final heating of the cathode which accompanies theexhaust and sealing-oil of the device in which the coated cathode ismounted. As a result, little or no heat treatment of the cathode isrequired in the tube assembly to put the cathode into activeelectron-emitting condition.

As a consequence of our invention, the time required to prepareelectrical discharge tubes. (commonly called "vacuum tubes" even when 2W V 2,m,aae 1. The method of providing a cathode with alkaline earthoxide which consists in applying to the surface thereof a coatingincluding alkaline earth hydroxide and thereupon dissof ciating. suchhydroxide intooxide and water vaper in the presence of a non-oxidizinggas. 7

2. The methodof providinga cathode with a coating of alkaline earthoxide whiclrconsistsin applying thereto alkaline earth hydroxide. andthereupondissociating. such hydroxide into oxide and water vapor in thepresence of hydrogen gas.

3. The metho of providing afoundation metal for a thermionic athode withan alkaline earth oxide which consists in applying thereto-a coatingincluding alkaline earth hydroxide which contains insufiicient oxidizingingredients to oxidize the surface of said metal and heating saidcoating to the dissociation temperature of said sivity of a cathode foran electrical discharge containing gas), that is, to properly degasifying of disintegrated coating material on other parts, such as the grid,is practically eliminated. The initial emission of oxidecoated cathodesis uniformly high, irregularities in composition being eliminated as aconsequence of our invention. It is more readily degasified thancathodes coated with activating material by other methods and hence isparticularly useful in television tubes where gases tend to produce ionspo Howe er. such cathodes can be advantageously used in rectifier-s,amplifiers, vapor lamps, including fluorescent lamps and other forms ofdischarge devices, including both vacuum devices and gas-filled devices.

The preparation of oxide coating in accordance with our inventionextends to both thermionic, or incandescent, and non-thermionic or cold"cathodes.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

device which consists in applying thereto a fusion containing alkalineearth hydroxide and thermally dissociating said hydroxide into oxide andwater vapor in the presence of a non-oxidizing gas at substantialpressure.

5. The method of coating a cathode of oxidizable metal with barium oxidewhich consists in bringing said cathode into contact with a fusion ofbarium hydroxide and oxide at a temperature of about 475 to 500 C., andheating said cathode to a temperature of about 650 to 800 C. in acurrent of hydrogen gas.

6. The method of providing an electrical discharge device with a cathodecoated with an oxide of an alkaline earth metal which consists in fusinga pellet comprising the hydroxide of said metal in contact with ametallic cathode foundation, thermally decomposing said hydroxide whileenveloped in hydrogen gas at substantially atmospheric pressure andfinally mounting said coated foundation metal in operative position insaid device.

7. The method of coating a cathode foundation of oxidizable metal withbarium oxide which consists in dipping said foundation into a fusioncomprising barium hydroxide and oxide maintained at about 475 to 500 C.,removing said foundation coated with a layer of said fusion and heatingto a temperature suillciently high to decompose said hydroxide whileenveloped in hydrogen at substantially atmospheric pressure.

RALPH J. BONDLEY. HERMAN A. LIEBI-IAFSKY. ARTHUR F. WIN-SLOW.

